[RFC PATCH v2 0/3] sched/fair: introduce new scheduler group type group_parked

Tobias Huschle
2025-02-17T11:33:14Z

Changes to v1

parked vs idle
- parked CPUs are now never considered to be idle
- a scheduler group is now considered parked iff there are parked CPUs
and there are no idle CPUs, i.e. all non parked CPUs are busy or there
are only parked CPUs. A scheduler group with parked tasks can be
considered to not be parked, if it has idle CPUs which can pick up
the parked tasks.
- idle_cpu_without always returns that the CPU will not be idle if the
CPU is parked

active balance, no_hz, queuing
- should_we_balance always returns true if a scheduler groups contains
a parked CPU and that CPU has a running task
- stopping the tick on parked CPUs is now prevented in sched_can_stop_tick
if a task is running
- tasks are being prevented to be queued on parked CPUs in ttwu_queue_cond

cleanup
- removed duplicate checks for parked CPUs

CPU capacity
- added a patch which removes parked cpus and their capacity from
scheduler statistics

Original description:

Adding a new scheduler group type which allows to remove all tasks
from certain CPUs through load balancing can help in scenarios where
such CPUs are currently unfavorable to use, for example in a
virtualized environment.

Functionally, this works as intended. The question would be, if this
could be considered to be added and would be worth going forward
with. If so, which areas would need additional attention?
Some cases are referenced below.

The underlying concept and the approach of adding a new scheduler
group type were presented in the Sched MC of the 2024 LPC.
A short summary:

Some architectures (e.g. s390) provide virtualization on a firmware
level. This implies, that Linux kernels running on such architectures
run on virtualized CPUs.

Like in other virtualized environments, the CPUs are most likely shared
with other guests on the hardware level. This implies, that Linux
kernels running in such an environment may encounter 'steal time'. In
other words, instead of being able to use all available time on a
physical CPU, some of said available time is 'stolen' by other guests.

This can cause side effects if a guest is interrupted at an unfavorable
point in time or if the guest is waiting for one of its other virtual
CPUs to perform certain actions while those are suspended in favour of
another guest.

Architectures, like arch/s390, address this issue by providing an
alternative classification for the CPUs seen by the Linux kernel.

The following example is arch/s390 specific:
In the default mode (horizontal CPU polarization), all CPUs are treated
equally and can be subject to steal time equally.
In the alternate mode (vertical CPU polarization), the underlying
firmware hypervisor assigns the CPUs, visible to the guest, different
types, depending on how many CPUs the guest is entitled to use. Said
entitlement is configured by assigning weights to all active guests.
The three CPU types are:
- vertical high : On these CPUs, the guest has always highest
priority over other guests. This means
especially that if the guest executes tasks on
these CPUs, it will encounter no steal time.
- vertical medium : These CPUs are meant to cover fractions of
entitlement.
- vertical low : These CPUs will have no priority when being
scheduled. This implies especially, that while
all other guests are using their full
entitlement, these CPUs might not be ran for a
significant amount of time.

As a consequence, using vertical lows while the underlying hypervisor
experiences a high load, driven by all defined guests, is to be avoided.

In order to consequently move tasks off of vertical lows, introduce a
new type of scheduler groups: group_parked.
Parked implies, that processes should be evacuated as fast as possible
from these CPUs. This implies that other CPUs should start pulling tasks
immediately, while the parked CPUs should refuse to pull any tasks
themselves.
Adding a group type beyond group_overloaded achieves the expected
behavior. By making its selection architecture dependent, it has
no effect on architectures which will not make use of that group type.

This approach works very well for many kinds of workloads. Tasks are
getting migrated back and forth in line with changing the parked
state of the involved CPUs.

There are a couple of issues and corner cases which need further
considerations:
- rt & dl: Realtime and deadline scheduling require some additional
attention.
- ext: Probably affected as well. Needs some conceptional
thoughts first.
- raciness: Right now, there are no synchronization efforts. It needs
to be considered whether those might be necessary or if
it is alright that the parked-state of a CPU might change
during load-balancing.

Patches apply to tip:sched/core

The s390 patch serves as a simplified implementation example.

Tobias Huschle (3):
sched/fair: introduce new scheduler group type group_parked
sched/fair: adapt scheduler group weight and capacity for parked CPUs
s390/topology: Add initial implementation for selection of parked CPUs

arch/s390/include/asm/smp.h | 2 +
arch/s390/kernel/smp.c | 5 ++
include/linux/sched/topology.h | 19 ++++++
kernel/sched/core.c | 13 ++++-
kernel/sched/fair.c | 104 ++++++++++++++++++++++++++++-----
kernel/sched/syscalls.c | 3 +
6 files changed, 130 insertions(+), 16 deletions(-)

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2.34.1